Sindarin is Tolkien’s Elvish language based largely on Welsh and spoken by many Elves in Middle-earth.
Tolkien designed Sindarin as part of a family of related Elvish languages. Its phonology and general sound were strongly influenced by Welsh, while its fictional history connected it to earlier forms of Elvish speech. Sindarin became the everyday Elvish language of many Elves in Middle-earth during the later ages.
Quenya is the main language readers commonly compare with Sindarin. Tolkien based much of Quenya’s sound and structure on Finnish, and it functioned as a more formal or literary language among many Elves. The two languages are related within Tolkien’s invented linguistic history but are not interchangeable.
Sindarin names include Sauron’s name in the Third Age, as well as place-names such as Minas Tirith and Lothlórien. Tolkien developed vocabulary and grammar for the language over many decades.